Films comprising polyesters in the stated thickness range are sufficiently well known. The disadvantage of unstabilized polyester films, however, is their tendency to hydrolyze, in particular at temperatures above the glass transition temperature of the respective polyester. Here, tendency to hydrolyze is understood as meaning the property of the polyester to undergo hydrolytic degradation under humid conditions, which is evident, for example, from a reduction of the IV or SV value. This is a limiting factor for the use of polyester films, particularly in applications with a relatively high thermal load, such as in film capacitors, cable sheathing, ribbon cables or engine-protection films, but also in long-term applications, such as in glazing and outdoor applications.
The tendency to hydrolyze is particularly pronounced in the case of aliphatic polyesters, but also in the case of aromatic polyesters, such as PBT and PET. If the tendency of PET to hydrolyze becomes too great for the application, it is necessary to rely on the somewhat more hydrolysis-stable PEN or even on other polymers, such as, for example, polyetherimides or polyimides. However, these are substantially more expensive than PET and, for economic reasons, are therefore often not a solution.
It has therefore already been proposed to improve the hydrolysis stability of polyester films by incorporating hydrolysis stabilizers.
The more hydrolysis-stable polyester raw materials which are obtained by using carbodiimides, and fibers and films produced therefrom, are known (U.S. Pat. No. 5,885,709, EP-A-0 838 500, CH-A-621 135). However, films which are produced from such raw materials tend to emit, in gaseous form, both in the production and in subsequent use, isocyanates and other byproducts and degradation products which irritate the mucous membrane or are harmful to health. This is a much greater problem in the case of sheet-like structures, such as a film having a large surface than in the case of injection-molded parts or the like.
Hydrolysis stabilizers based on epoxy groups also lead to hydrolysis stabilization and are described, for example, in EP-A-0 292 251 or U.S. Pat. No. 3,657,191. However, these compounds are based on the production of oxirane rings by means of epichlorohydrin and, inter alia owing to their terminal epoxy groups, tend to eliminate low molecular weight toxic compounds on heating, so that problems similar to those with the use of carbodiimides are associated with the use of these substances. Moreover, their incorporation into the polyester matrix is insufficient, which leads to long reaction times and, in the case of oriented polyester films, to a considerable undesired haze.
Moreover, known hydrolysis stabilizers, such as carbodiimides and other substances, such as those described in EP-A-0 292 251, have the disadvantage that in some cases they lead to considerable increases in molecular weight (increase in viscosity) in the polymer during extrusion and thus make the extrusion process unstable and difficult to control.